1. Field of the Invention
The present invention relates to an imaging apparatus that includes an image sensor.
2. Description of the Related Art
Complementary metal-oxide-semiconductor (CMOS) image sensors have been recently often used in digital single lens reflex cameras and video cameras. These CMOS image sensors need to have an increased number of pixels, faster imaging speed, and greater ISO (higher sensitivity). A CMOS sensor having such a high number of pixels is very effective in imaging high definition still images. Recently the number of pixels has exceeded ten million. On the other hand, the number of pixels required for capturing a moving image is less than the number of pixels required for capturing a still image, and is about 300,000 for a normal moving image. Even for a moving image supporting the high vision of a full high-definition (HD) standard, this number is about two million pixels. Further, the frame rate is about 30 frames/second or 60 frames/second.
When capturing a moving image using a CMOS sensor with many pixels that is designed to capture still images, from a pixel number and frame rate perspective, it is typical to perform pixel thinning processing or pixel addition processing. Further, to read the pixel signals at high speed, a method in which only the areas of the image to be used are read by performing skip-reading is also employed. Generally, the imaging plane of a CMOS sensor is configured with a 3:2 aspect ratio. However, since the aspect ratio for high vision is 16:9, in high vision moving image mode, the top and bottom portions of the sensor are not used. Consequently, if the pixel signals are read while skipping the areas that are not used, the reading time can be shortened.
Further, as discussed in Japanese Patent Application Laid-Open No. 2009-17517, for example, there are also imaging apparatuses that include a mode for performing an enlarged display without reducing resolution by cutting the image at an arbitrary position in the imaging screen. In this case, a high speed display of 30 frames/second can be performed by skip-reading only the pixel signals in an arbitrary area, rather than reading the pixel signals of all of the pixels in the image sensor.
Usually, to obtain a signal (a black reference signal) serving as a signal level reference, an image sensor includes an optical black region (OB region) formed from a plurality of shielded optical black pixels (OB pixels) which do not to react to light. An HOB region is provided on the left side of the effective pixel region and a VOB region is provided on an upper portion of the effective pixel region. The calculation processing of the effective pixel signals is performed based on the signal level obtained at the optical black region by clamping the output signals of the OB pixels at a predetermined level. Specifically, dark current resulting from exposure for a long time and shading in the vertical direction of a dark-time signal can be corrected.
The operation for clamping OB pixel output at a predetermined level will now be described. Such an operation is discussed in, for example, Japanese Patent Application Laid-Open No. 2008-41776. First, while monitoring the signal output of the VOB region, a clamping operation is performed by providing feedback at a large gain so that the output signal rapidly reaches a predetermined target level (high speed clamping). Then, when clamping has been performed up to the target level, the monitoring region is moved to the HOB region, and the clamping operation is continued by providing feedback at a small gain so that the output signal for the HOB region slowly reaches a predetermined target level (low speed clamping). Thus, shading in the vertical direction can be corrected by performing low speed clamping for the HOB region.
However, when performing the drive for skip-reading as described above, if the vertical direction shading is large, an offset step is produced at the boundary between before and after skipping. If the low speed clamping is performed without considering the offset step, the clamp operation to the predetermined level may not be in time, so that unfinished corrections may remain at the upper portion of the screen.